# Thread: How does a GEC limit overvoltage from lightning and grid surges?

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Originally Posted by kwired
Remember Ohm's law?

If you have a 1000 volt source and place both a 10 ohm and a 10,000 ohm resistor in parallel across the source doesn't both carry current? One happens to carry much more current than the other but both are a part of a closed circuit with a voltage across them.

Same reason a 1500 watt heater and a 5 watt lamp both operate when connected in parallel to the same voltage source.
agreed.

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Originally Posted by electrofelon
No disagreement. But how does this reduce shock potential? Unless its a real high impedance source (such as capacitance, or you are standing pretty much right on the electrode, the other path doesnt change anything about the path shocking the person.
Let's say there is a 1 ohm path and a 10000 ohm path...and 100V. Path one is 100A. Path two is 10 milliamps.

Now let's say a 1000 ohm human finds himself/herself in series with each of those paths.

Path 1 is now 1001 ohms (1000 for the human and 1 for the original path). The current, within a decimal or two, is now 100 milliamps and the voltage is still 100. The voltage will be proportional to the resistances along the path. Within a decimal or two, the portion across the human is 100 milliamp at 100V.

Path 2 is now 11000 ohm (1000 for the human and 10000 for the original path). The current is now about 9 milliamps. Within a decimal or two the portion across the human is 9 milliamps at 9V.

A human in series with Path 1 is in trouble. A human in series with Path 2....probably OK.
Last edited by JPinVA; 11-08-18 at 02:13 PM.

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Originally Posted by PetesGuide
I'm trying to get a deeper and accurate understanding of how a Grounding Electrode Conductor works.

What is most confusing to me is that I know a GEC doesn't help with ground faults from hot to the safety ground path--so if it doesn't help in that case, how does it lower the voltage from lightning strikes, unintentional contact with high-voltage lines, grid surges from switching feeders, etc.?
One of the most common code violations I've seen throughout dozens of chemical plants/refineries is the absence of a GEC on Delta-Wye transformers.
The vast majority of ground fault will travel via the EGC within the circuit conductors - see NEC 250.134(B) handbook - however a small percentage will divide into the GEC at the XO. A lightning strike that hits the earth (nearby) will most likely also hit the GEC as well and travel into the XO.

4. Originally Posted by JPinVA
Let's say there is a 1 ohm path and a 10000 ohm path...and 100V. Path one is 100A. Path two is 10 milliamps.

Now let's say a 1000 ohm human finds himself/herself in series with each of those paths.

Path 1 is now 1001 ohms (1000 for the human and 1 for the original path). The current, within a decimal or two, is now 100 milliamps and the voltage is still 100. The voltage will be proportional to the resistances along the path. Within a decimal or two, the portion across the human is 100 milliamp at 100V.

Path 2 is now 11000 ohm (1000 for the human and 10000 for the original path). The current is now about 9 milliamps. Within a decimal or two the portion across the human is 9 milliamps at 9V.

A human in series with Path 1 is in trouble. A human in series with Path 2....probably OK.
Your example doesn't make sense for two reasons: first, the resistance values are skewed too far out of the realistic range. Second, it is very unlikely a person would find themselves in series with a fault.

5. Originally Posted by PetesGuide
I'm trying to get a deeper and accurate understanding of how a Grounding Electrode Conductor works.

What is most confusing to me is that I know a GEC doesn't help with ground faults from hot to the safety ground path--so if it doesn't help in that case, how does it lower the voltage from lightning strikes, unintentional contact with high-voltage lines, grid surges from switching feeders, etc.?
Here, it is stops the system from floating to some indeterminate voltage wrt ground.

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Originally Posted by JPinVA
...
...The current varies proportional to each path's resistance/impedance of the path. In our example, both paths are taken, but 100A flows on one path and 1A flows on the other...a significant difference.
Not just to be picky, but to hopefully add to the understanding, to be correct the current varies in inverse proportion to the resistance.
Another way of saying the same thing is that the current varies proportionally to the conductance/admittance of the path. (Since conductance is the inverse of resistance and admittance is the inverse of impedance. They used to be measured in Mhos (Ohms backwards), but now have been systematized to the unit of Siemens.)

7. I suppose earthing does provide some reduction in potential difference for a small item, like a lamp post, where anytime you would be touching the lamp post, you would be very close to the electrode. There is a good diagram I cant find, maybe someone will post, that shows the voltage gradient as you move away from a typical electrode.

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Originally Posted by electrofelon
Your example doesn't make sense for two reasons: first, the resistance values are skewed too far out of the realistic range. Second, it is very unlikely a person would find themselves in series with a fault.
Actual values are not pertinent to explaining the existence of the difference, nor is the liklihood of the event. I used the assumptions to make a quantitative analysis to show the difference.

I can also explain things in quantitative ways. In our charged world, the things around us become charged. Charged, as in containing different values of positive or negative charges with respect to ground. Without a low impedance path to earth, the charges can build up to the point of a very high potential difference (between ground and the objects so charged). And the bigger the object(s) so charged, the bigger the aggregate number of charges.

The only thing stopping the charges from getting to ground is the lack of a path. If the potential gets high enough, it will eventually exceed the voltage necessary to overcome the dielectric properties of the things between these charges and the earth. And when that moment comes....ZAP!

As a human has dielectric properties of less impedance than most high impedance "things", human interaction in the charged environment can often close the dielectric gap between the build up of charges and ground. The human becomes the pathway for the built up charges to reach ground.

Note this is similar to...but not the same as...those static discharges we get on cold winter mornings. Our morning zaps are due to charges that have built up on ourselves...so the flow is limited to that equalization....it's not through us...but from/to us. When equipment (or a building) builds up charges...the amount of charge gets ever bigger as the equipment or building gets bigger. These can be of sufficient quantity to be lethal when passing through a human that "closes the connection" to ground. Or, they can simply cause a fire or an explosion if the zap is sufficient to start something burning or initiate an explosive reaction.

Now...introduce the GEC and associated bonding. The existence of that ground connection allows those built-up charges to bleed away into ground in an orderly fashion. It prevents the build up of charges to dangerous levels. Directly dangerous, as when a human becomes part of the circuit (or, more precisely, simply the path required to equalize the charges)....or indirectly dangerous as when the zap causes a fire or an explosion.

Note that there are indeed instances when even the GEC isn't sufficient to protect someone. A lightning event, even an indirect event, can deposit charges more rapidly over a short duration than the GEC and bonding can handle. This raises the possibility of the human once again closing the air gap during such an event. But all said, a GEC, even though it can't prevent this from happening in all instances, will still provide a measure of protection (better than nothing) during the event and a larger measure of protection as it equalizes the charges after the event.

9. Originally Posted by electrofelon
I suppose earthing does provide some reduction in potential difference for a small item, like a lamp post, where anytime you would be touching the lamp post, you would be very close to the electrode. There is a good diagram I cant find, maybe someone will post, that shows the voltage gradient as you move away from a typical electrode.
Is this the one?

10. Originally Posted by ActionDave
Is this the one?

Yup, thanks. Helps a little, not much, and you've got to be very close to the electrode.